1. Field of the Invention
This invention relates to a fluidized bed apparatus, such as a particle coater/granulator equipped with a plurality of replaceable filter cartridges which filter the air or gas being discharged from the apparatus. The cartridges are arranged in a divided filter housing. The flow of outlet gas, or process gas, from the fluidized bed is effectively bypassed through a particular cartridge or group of cartridges during a cartridge cleaning operation which is effected by the discharge of high pressure air or gas within the interior of the filter cartridge.
2. Description of Related Art
Various different forms of air filtering systems including bag type systems and cartridge type systems heretofore have been provided such as those disclosed in U.S. Pat. Nos. 4,588,366 and 4,953,308.
With the filter system such as that disclosed in U.S. Pat. No. 4,588,366, a plurality of filter elements must be incorporated for each air filter assembly and the most effective manner of cleaning such a filter system involves mechanical means for effecting vibration of the filter elements during a cleaning operation. The mechanical means which must be provided for vibrating the filter elements during a cleaning operation involves considerable expense and requires a reasonable degree of maintenance.
Filter cartridge systems of the type described in U.S. Pat. No. 4,953,308 utilize high pressure internal air for periodic cleaning of the air filter cartridges. These systems are inefficient in that the multiple filter cartridges are mounted in an undivided upper filter housing and the cleaning of one filter cartridge by the discharge of high pressure air therewithin occurs without interrupting the fluidized bed airflow. This results in inefficient cleaning of the filter surfaces. The discharge of particles from the filter cartridge requires the particles to discharge against a counter-current air flow. These particles are typically the fine particles which have the least chance of resisting the flow of air from the fluidized bed. As a result, to the extent these fine particles are dislodged, they will be drawn back into the cartridge from which they came. Because these particles do not get reintroduced into the fluidized bed, the efficiency of the process is diminished. For example, during an agglomeration process, because some of the fine particles do not get reintroduced, these unagglomerated particles could result in a product having content uniformity problems. Similarly, in a coating process, the particles that do not get reintroduced will result in a product having uncoated particles that will compromise the product performance. Moreover, those particles that do get blown from the exterior of such filter cartridge may be drawn to the exterior of an adjacent filter cartridge which remains in operation, thereby overloading the operational filter cartridge with particles from the cartridge being cleaned. This results in the cartridges having to be cleaned more frequently, which is expensive, as well as having to change the filters more frequently.